This R21 proposal is focused on enhancing the scientific capabilities of the Xiphophorus genetic model by construction and validation of transgenic fish lines harboring a system capable of mobilizing the SB transposon. At least two transgenic Xiphophorus lines will be produced: 1) Xiphophorus maculatus Jp 163 A containing the SB transposon with associated reporter gene expression; and 2) a second line in the same genetic background containing the gene encoding SB transposase under control of a strong constitutive promoter. The SB transposon itself cannot transpose in the absence of the transposase protein. Thus, interbreeding these two transgenic lines is expected to result in a burst of transposon mediated insertional mutagenesis. Once mutagenized, these hemizygous transposase/transposon animals will be used as parents in interspecies crosses. F1 interspecies hybrid animals that have not inherited the transposase (i.e. 50% of the F1 offspring) will be scored for modulation of phenotypic expression for several complex multigenic traits. Due to the diversity of Xiphophorus species, the transgenic system to be produced will be unparalleled in genetic power to identify candidate genes involved in complex multigenic traits that may then be confirmed using classical genetic approaches.